Ethylene vinyl alcohol (EVOH) copolymers are used widely for packaging food, medical solutions and other products. Often the function of EVOH is to provide a barrier to passage of gases such as oxygen through the packaging. There is a need in industry to improve the toughness of films containing EVOH, especially at lower temperature. For example, EVOH-based multilayer films are used in packaging meat, which is often stored and transported at freezer temperatures.
There are approaches in toughening EVOH with different polymeric modifiers with varied results. Adding typical modifiers that could bring about a desirable toughness and stiffness tend to reduce the optical clarity and can turn EVOH into an opaque film. Blends of EVOH and modifiers typically consist of microscopic particles of one polymer dispersed in a continuous phase of the other polymer. Poorly dispersed and/or large particles tend to scatter rather than transmit light. As a result the polymer blends tend to be opaque. For many food packaging and health care applications, the see-through clarity and/or contact clarity of a film or structure, either monolayer or multilayer, may be important. These applications demand new materials with good processability, mechanical properties, impact toughness, or optical properties, or be available at an affordable cost for broad applications. Ionomeric resins (ionomers) are thermoplastic resins that contain metal ions in addition to organic-chain molecules. Ionomers have solid-state properties characteristic of cross-linked polymers and melt-fabricability properties characteristic of uncrosslinked thermoplastic polymers (see, e.g., U.S. Pat. No. 3,264,272). Ionomers may be prepared from copolymers of an olefin such as ethylene with an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, and/or other acids, and optionally softening comonomers such as alkyl acrylates or alkyl methacrylates. At least one alkali metal, transition metal, or alkaline earth metal cation, such as lithium, sodium, potassium, magnesium, calcium, or zinc, or a combination of such cations, is used to neutralize some portion of the acidic groups in the copolymer. Owing to their water-like clarity and high toughness, ionomers such as those available from E. I. du Pont de Nemours and Company (DuPont) under the trademark SURLYN have been disclosed for use in packaging.
Ionomers have been used to modify EVOH. For example, U.S. Pat. Nos. 6,214,392 and 6,333,061 and US Patent Application 2006/0228503 disclose EVOH modified with polyamide-containing ionomers (N-ionomers).
Recently a new family of ionomers has been disclosed in U.S. Pat. No. 5,700,890, wherein neutralized ethylene acid copolymers are prepared using dicarboxylic acids, or derivatives thereof, as monomers in addition to the monocarboxylic acids used in typical ionomers. These “anhydride ionomer” copolymers may further contain an alkyl acrylate comonomer.
US Patent Applications 2005/0203253, 2005/020762 and 2006/0142489 disclose compositions wherein polyamides are toughened with anhydride ionomers.
As disclosed in this application, films made from anhydride ionomer-modified EVOH show excellent impact resistance (more than 100% better than unmodified EVOH) and unlike EVOH modified with N-ionomers, almost no change in oxygen barrier. Film optical properties such as haze and transparency also show almost no change compared to an unmodified sample. Anhydride ionomer-modified EVOH bonded to PE with an adhesive tie resin has similar adhesion to that exhibited by N-ionomer-modified EVOH. Films and other shaped articles comprising anhydride ionomer-modified EVOH can be used in both food and non-food packaging where high barrier, toughness and clarity are desired.